1. Field of the Invention
The present invention relates to an exchange coupling film for fixing a magnetization direction of a ferromagnetic body, a magnetoresistance effect device incorporating the same which causes a substantial magnetoresistance change with a low magnetic field, a magnetoresistance head incorporating the same which is suitable for use in high density magnetic recording and reproduction, and a method for producing such an exchange coupling film.
2. Description of the Related Art
In recent years, the density of hard disk drives has been dramatically increased, while reproduction magnetic heads have also been improved dramatically. Among others, a magnetoresistance effect device (hereinafter, referred to simply as an "MR device") utilizing a giant magnetoresistance effect, which is also called a "spin valve", has been researched actively and is expected to have the potential to significantly improve the sensitivity of a currently-employed magnetoresistance effect head (hereinafter, referred to simply as an "MR head").
A spin valve includes two ferromagnetic layers and a non-magnetic layer interposed between the ferromagnetic layers. The magnetization direction of one of the ferromagnetic layers (hereinafter, referred to also as a "pinned layer") is fixed by an exchange bias magnetic field from a pinning layer (the ferromagnetic layer and the pinning layer are referred to collectively as an "exchange coupling film"). The magnetization direction of the other one of the ferromagnetic layers (hereinafter, referred to also as a "free layer") is allowed to change relatively freely in response to an external magnetic field. In this way, the angle between the magnetization direction of the pinned layer and that of the free layer is allowed to change so as to vary the electric resistance of the MR device.
A spin valve film has been proposed which utilizes Ni-Fe for the ferromagnetic layer, Cu for the non-magnetic layer and Fe-Mn for the pinning layer. The spin valve film provides a magnetoresistance rate of change (hereinafter, referred to simply as an "MR ratio") of about 2% (Journal of Magnetism and Magnetic Materials 93, p. 101, (1991)). When Fe-Mn is used for the pinning layer, the resulting MR ratio is small, and the blocking temperature (a temperature at which the pinning layer provides no effect of fixing the magnetization direction of the pinned layer) is not sufficiently high. Moreover, the Fe-Mn film itself has a poor corrosion resistance. In view of this, other spin valve films have been proposed which employ pinning layers with materials other than Fe-Mn.
Among others, a spin valve film which employs an oxide, such as NiO or .alpha.-Fe.sub.2 O.sub.3, for the pinning layer is expected to realize a dramatically large MR ratio of about 15% or greater.
However, the blocking temperature of NiO is not sufficiently high. Therefore, the thermal stability of the MR device employing NiO is undesirable.
When a spin valve film employs a pinning layer of .alpha.-Fe.sub.2 O.sub.3, on the other hand, the reverse magnetic field of the pinned layer is not sufficiently large when the pinning layer is thin. Particularly, a spin valve film having a dual spin valve structure or a spin valve film where an .alpha.-Fe.sub.2 O.sub.3 layer is formed on the pinned layer has a strong tendency that the reverse magnetic field of the pinned layer obtained in the overlying .alpha.-Fe.sub.2 O.sub.3 layer is insufficient. Moreover, the thermal stability of the .alpha.-Fe.sub.2 O.sub.3 -employing spin valve film is also undesirable for the same reasons as the NiO-employing spin valve film. Furthermore, the .alpha.-Fe.sub.2 O.sub.3 -employing spin valve film has other problems in controlling the anisotropy during deposition in a magnetic field or during a low-temperature heat treatment in a magnetic field.